The present invention relates to a continuous process for the preparation of polytetramethylene adipamide (nylon 4,6) by heating the salt of 1,4 diaminobutane and adipic acid at elevated pressure and subjecting the prepolymer thus formed to after-condensation in at least one subsequent reaction zone.
Several non-continuous processes for preparing polytetramethylene adipamide are known from a number of publications. European patent application laid open to public inspection No. 39524 describes a process for the preparation of polytetramethylene adipamide in which the salt of 1,4 diaminobutane and adipic acid is converted into a prepolymer under carefully defined conditions and this prepolymer as a solid is subjected to after-condensation in a second reaction zone. A similar process is disclosed in European patent application laid open to public inspection No. 38094.
Japanese patent specification No. 17036/74 discloses a process for the preparation of nylon 4,6 in which a solid product with a relative viscosity (.eta..sub.rel, measured at 1 gram polyamide in 100 ml of 96 wt. % concentrated H.sub.2 SO.sub.4 at 20.degree. C.) of at most 2.37 is obtained by reacting at varying temperatures.
U.S. Pat. No. 4,460,762 discloses a process for the preparation of nylon 4,6 polyamide by forming a polyamide prepolymer by the condensation reaction between 1,4-diaminobutane and adipic acid, and then after-condensing the prepolymer in the solid phase, under a water-vapor containing atmosphere.
U.S. Pat. No. 4,408,836 discloses a process for preparing nylon 4,6 polyamide by forming in a separate first stage a polyamide prepolymer by heating a salt of 1,4 diaminobutane and adipic acid, under a water vapor containing atmosphere, then in a separate second stage, after-condensing the polyamide prepolymer under a water-vapor containing atmosphere.
U.S. Pat. No. 4,446,304 discloses shaped articles molded from nylon 4,6 polyamide and a process for the production of these articles.
U.S. Pat. No. 4,463,166 disclose for preparing nylon 4,6 polyamide in a non-aqueous inert polar solvent. Higher molecular weight polyamides may be obtained following after-condensation.
None of these processes discloses a continuous operation.
For the preparation of nylon 6,6 continuous processes are known. German patent specification No. 2,417,003 discloses a process in which the salt of 1,6 diaminohexane is heated in a first reaction zone until at least 80% polycondensation is achieved, is subsequently expanded adiabatically in a second zone and, is reheated within a very short time in a third zone.
In the Netherlands patent application No. 6,603,078 laid open to public inspection, a process is described for the preparation of polyamide by pumping a salt of a diamine and dicarboxylic acid (according to the example hexamethylene adipamide) through a heated tube.
U.S. Pat. Nos. 3,900,450, 3,948,862 and 3,860,820 disclose processes for continuous preparation of nylon 6,6.
Practice has shown it is impossible to prepare nylon 4,6 with a high molecular weight and a good color (white) while using the known conditions used for producing nylon 6,6. The object of this invention is to provide a continuous process for the preparation of white nylon 4,6 with a high molecular weight.